Studies are being conducted for a dereverberation technology for highly accurately removing, from the sound in a microphone, reverberation components resulting from the reflection of sound by, e.g., walls or the roof of a room, and for picking up only a component that directly reaches the microphone from the speaker's mouth (direct sound) with high accuracy (see Non-Patent Literature 1, for example).
FIG. 1 schematically illustrates a sound propagation process in a room. As illustrated in FIG. 1, the sound emitted from a speaker's mouth and received by a microphone can be largely categorized into a direct sound component which is a component propagating from the speaker's mouth to the microphone in the shortest route (see FIG. 1(i)), and a reverberation component which is a sound component having been reflected or bounced by walls or the floor of a room, or a person in it, before entering the microphone.
According to conventional dereverberation technology, the sound entering the microphone is estimated using an inverse filter of the propagation characteristics as a linear filter, and the estimated linear filter is superimposed on the microphone input signal. In this way, the technology divides the microphone input signal into the direct sound component and the reverberation component, thus extracting only the direct sound component without reverberation. Conventionally, the reverberation component has been considered to be non-fluctuating where the propagation route is not varied over time (see FIG. 1(ii)).
However, in reality, in addition to the non-fluctuating reverberation component, there is a fluctuating reverberation component (see FIG. 1(iii)) which is a component that is reflected by persons (such as their heads), e.g., moving in the room before being transmitted to the microphone. The fluctuating reverberation component may be considered a component that cannot be sufficiently erased by the linear filter. For removing such fluctuating reverberation component, use of a non-linear filter is proposed (see Patent Literature 1, for example).